Modern photolithography equipment is based on optical lithography, which uses optics to accurately project and expose a reticle or mask pattern on a photoresist-covered wafer. Photolithography is an important part of the wafer fabrication process, and by some estimates may consume up to sixty percent of the wafer's fabrication time. Photolithography has been widely used in the semiconductor industry for more than fifty years to form a wide range of structures, such as vias, conductive lines, and other structures commonly present in integrated circuit devices. Integrated circuits are what power many of today's consumer electronics and they can be found in cellphones, video cameras, portable music players, computers, and even automobiles.
Generally, the photolithography process and its corresponding equipment consist of a light source transmitted through an optical system onto a reticle or mask with a pattern. The pattern produced by the light is then aligned to a wafer covered with a light-sensitive photoresist by an alignment system, wherein the pattern is then transferred to the photoresist.
More specifically, the photolithography process can begin with the formation of a photoresist layer on or over the top surface of a semiconductor substrate or wafer. A reticle or mask with a circuit design or pattern defined by opaque regions, which are often formed of chrome, and clear regions, which are often formed of silica, is then positioned over the photoresist coated wafer. Commonly, multiple reticles or mask patterns are employed to attain the final circuit pattern on the wafer surface.
Each one of the masks or reticles is placed between a light source and a projection lens system. The pattern produced by the light transmitted through the mask or reticle is then focused to generate a reduced mask image on the wafer. The focusing and reduction of the mask image pattern is typically done using the projection lens system, which contains one or more lenses, filters, and/or mirrors. The light passing through the clear regions of the reticle or mask exposes the underlying photoresist layer and depending upon the photoresist layer composition, the exposed portions of the photoresist can either become soluble or insoluble to a subsequent developer. This patterned photoresist layer is then used to remove or further process exposed portions of underlying structural layers within the wafer. The end result is a semiconductor wafer coated with a photoresist layer exhibiting a desired pattern, which defines geometries, features, lines and shapes of the reticle or mask pattern.
The resolution achieved through photolithography depends, in part, on the wavelength and coherence of the light source, as well as, the numerical apertures (NA) of the lens within the photolithography system. As the critical dimension geometry of each new successive integrated circuit generation decreases, the resolution (i.e.—the ability to discretely discern pairs of closely spaced features on a wafer) of the corresponding photolithographic equipment must also improve. Although, the resolution of photolithographic equipment can be improved by using a lens with a higher NA, it unfortunately comes at a cost because the depth of focus of a lens is inversely proportional to the square of the NA. Consequently, improving the resolution of a system by increasing the NA reduces the depth of focus of the system. Poor depth of focus will cause some features of the wafer to be out of focus, which leads to poor exposure of the wafer to the reticle image pattern. Thus, proper design of any photolithography equipment must consider the compromise between resolution and depth of focus.
Commonly, photolithographic equipment is called upon to form a square contact hole from a square feature, but as photolithography equipment expands into the sub-wavelength realm, a circular contact hole is frequently formed by the square feature due to the corner-rounding effect at each corner. Furthermore, as the critical dimensions of these square contact holes or circular contacts holes continues to decrease, the dimensions of the square features forming them become too small to transmit the energy needed to fully expose the photoresist. Consequently, the square features are sized up on design to permit the transmission of the necessary energy to fully expose the photoresist. But, due to the physical constraints imposed by lens manufacturing capabilities, the size up of these square features has a finite limit and such size up accommodations have their limits as the technology node for integrated circuit design continues to decrease to meet consumer demand.
Advanced photolithography has made use of optical enhancement techniques, such as, phase shifting masks and optical proximity correction methods to improve the resolution of smaller features as the technology node continues to shrink. But these techniques also have their limits as the technology node for integrated circuit design continues to decrease.
Thus, a need still remains for a reliable integrated circuit system and method of fabrication, wherein the integrated circuit system exhibits enhanced pattern resolution. In view of the ever-increasing commercial competitive pressures, increasing consumer expectations, and diminishing opportunities for meaningful product differentiation in the marketplace, it is increasingly critical that answers be found to these problems. Moreover, the ever-increasing need to save costs, improve efficiencies, and meet such competitive pressures adds even greater urgency to the critical necessity that answers be found to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.